Question: Luis ate 5 slices of cake. Christopher ate 4 slices. If Luis ate $\dfrac{5}{13}$ of the cake, what fraction of the cake is remaining?
If 5 slices represent $\dfrac{5}{13}$ of the cake, there must have been a total of 13 slices. $\text{fraction of cake remaining} = \dfrac{\text{number of slices remaining}}{\text{number of slices total}}$ Together they ate 9 slices, which leaves 4 out of 13 slices remaining. There is $\dfrac{4}{13}$ of the cake remaining.